Agricultural machinery is operated under a variety of conditions. As these conditions change, it may be desirable to change the sensitivity of the steering. Steering valves are available with dual displacements, for example, to provide a selectable steering rate. The rate could be selectable as an operator preference or could be automatically selected as a function of a condition such as transmission gear selection or travel speed.
A variable steering rate would be particularly advantageous on a windrower. The dual path hydrostatic drive typically used on windrowers gives them unique handling characteristics. Operator preferences for steering rate vary considerably and may change as driving experience is gained. In addition, because the rate of wheel speed change (the speed of one wheel increases while the speed of the other wheel decreases) used for steering is relatively constant throughout the available range of ground speeds, the effective steering rate changes with ground speed. It may be desirable to optimize this inherent steering rate change. Lastly, it may be possible to enhance the steering if the rate could be electronically controlled. For example, it may be desirable to have a slightly faster steering rate at turn initiation to help overcome the inertial resistance to turn inherent in the dual path hydrostatic transmission steering arrangement.
Variable steering and all of its advantages are readily available on a windrower with an electro-hydraulic (EH) controlled ground drive. Components are readily available but such a steer-by-wire system has the disadvantage of requiring a level of redundancy to comply with safety expectations. This redundancy adds cost and complexity.
One type of primary steering valve that is widely used and therefore cost effective is steering wheel controlled and includes a spool located within a sleeve used together with a fluid meter. One drawback of this known type of primary steering valve is that internal leakage is a characteristic of these valves, with leakage resulting in lost fluid output impacting steering responsiveness under some operating conditions. While this leakage characteristic can be overcome by selecting a larger valve so that the necessary volume of fluid for the desired steering responsiveness or rate is provided by the valve, this solution would require larger volume components with added costs and space challenges for the entire system.
Thus, what is desired is a relatively simple and low cost steering control system with an electronically controlled steering rate, but which avoids the aforementioned disadvantages of the known mechanical steering control system.